Recently, solid electrolytes are used more often than the conventional electrolyte (aqueous) solutions. This is because firstly those solid electrolytes have good processability in application in electric and electronic components, and secondly there are trends for overall size and weight reduction of such components and further for power saving.
Proton conductive materials, both inorganic and organic, are known in the art. However, inorganic proton conductive compounds, such as uranyl phosphate hydrate, come with many difficulties when superposed as a conductive layer on a substrate or an electrode. For example, sufficient contact cannot be achieved in the interface between the conductive layer and the substrate or the like.
On the other hand, the organic proton conductive compounds include organic polymers that belong to the so-called cation exchange resins such as sulfonated vinyl polymers (ex. polystyrene sulfonic acid), perfluoroalkylsulfonic acid polymers represented by Nafion® (DuPont) and perfluoroalkylcarboxylic acid polymers, and that are obtained by introducing sulfonic or phosphoric groups in heat resistant polymers such as polybenzimidazole and polyether ether ketone (see, for example, Polymer Preprints, Japan, Vol. 42, No. 3, p. 730 (1993); Polymer Preprints, Japan, Vol. 42, No. 7, p. 2490-2492 (1993); and Polymer Preprints, Japan, Vol. 43, No. 3, p. 735-736 (1994)).
These organic polymers are generally used in the form of film. Their solvent solubility and thermoplasticity enable them to form a conductive membrane jointly on an electrode. However, many of the organic polymers are still insufficient in proton conductivity. In addition, they have poor service durability, reduce the proton conductivity at high temperatures (100° C. or above), have high humidity dependence, and are unsatisfactory in adhesion with the electrode. Further, because of the water-containing structure of these polymers, the conductive membranes are excessively swollen during operation, resulting in lowered strength and deformation. Accordingly, the organic polymers cause many problems in application to the electric and electronic components.
U.S. Pat. No. 5,403,675 proposes a solid polymer electrolyte comprising a sulfonated rigid-rod polyphenylene (namely, a sulfonated compound based on a polyarylene structure). This polymer mainly contains a polymer composed of phenylene chains that is obtained by polymerization of an aromatic compound, and the polymer has been sulfonated by reaction with a sulfonating agent to introduce the sulfonic groups. Although increasing the amount of the sulfonic groups introduced improves the proton conductivity, it also results in remarkably deteriorated mechanical characteristics such as toughness (for example, break elongation and folding resistance) and hot water resistance of the sulfonated polymer.